Control for a tunable laser

ABSTRACT

A control ( 20 ) and a method of controlling a tunable laser having a gain section ( 4 ), a phase change section ( 5 ) and a segmented Bragg grating reflector section ( 6 ) comprising a series of grating units ( 9 - 17 ) each of a different pitch, and an electrode ( 9   a - 17   a ) associated with each grating so that an electrical current is applicable to each individual grating. The control ( 20 ) includes a plurality of digitally controlled sources ( 31, 32 ) of electrical current which are each connected to switch means ( 33  or  35, 36 ) operable to independently connect each of said sources ( 31, 32 ) to a respective electrode (e.g.  11   a  or  12   a ) associated with a grating (e.g.  11  or  12 ) which is one of a subset of consecutive gratings (e.g.  11  &amp;  12 ) selected from said series of grating units ( 9 - 17 ).

RELATED APPLICATIONS

This application is a 35 U.S.C. 371 national stage filing ofInternational Application No. PCT/GB03/00863, filed 28 Feb. 2003, whichclaims priority to Great Britain Patent Application No. 0204668.8 filedon 28 Feb. 2002, in Great Britain. The contents of the aforementionedapplications are hereby incorporated by reference.

FIELD

This invention provides a control for a tunable laser particularly foruse in telecommunication systems.

BACKGROUND OF THE INVENTION

Tunable lasers for use in optical communications systems, particularlyin connection with wavelength division multiplex (WDM) telecommunicationsystems, are known. A known tunable system comprises stacks of singlewavelength distributed Bragg reflectors (DBR) lasers, which can beindividually selected; or tuned over a narrow range, or by a wide rangetunable laser that can be electronically driven to provide thewavelength required. Limited tuning range tunable lasers that rely uponthermal effects for tuning are also known.

Atypical optical fibre telecommunications band is the 1550 nm C-band,located in the infra-red spectrum with International TelecommunicationUnion (YrU) 200, 100 or 50 GHz channel spacing (the so-called ITU Grid)spread between 191 THz and 197 THz.

U.S. Pat. No. 4,896,325 discloses a wavelength tunable laser havingsampled gratings at the front and rear of its gain region. The laserdescribed in that specification relies on the use of two differentgratings which produce slightly different reflection combs in the frontand rear gratings. These provide feedback into the device. The gratingscan be current tuned in wavelength with respect to each other.Co-incidence of a maximum from each of the front and rear gratings isreferred to as a supermode. To switch the device between supermodesrequires a small incremental electrical current into one of the gatingsto cause a different pair of maxima to coincide in the manner of avernier. By applying electrical currents to the two gratings so that thecorresponding maxima track, continuous tuning within a supermode can beachieved.

Alternative forms of electrically tunable comb reflection gratings arecommonly known as superstructure gratings such as described in“Broad-Range Wavelength-Tunable Superstructure Grating (SSG) DBRLasers”, Y Thomori et al, IEEE Photonics Technical Letters, Volume 5,No. 6, Jul. 1993.

The applicant's co-pending UK application 0106790.9 discloses a tunablelaser having a gain section, a phase section and a segmented Bragggrating reflector comprising a series of grating units, at least two ofwhich have a different pitch, wherein an electrical current isapplicable to the grating having a longer pitch such that the wavelengthof the longer pitch grating can be tuned to the wavelength of theshorter pitch grating.

Another tunable laser is disclosed in the applicant's copending UKapplication 0118412.6 in which there is disclosed a tunable laserincluding a gain section bounded at one end by a first reflector in theform of a distributed Bragg grating reflector producing a first comb ofreflective peaks and on the other end by a second reflector in the formof a segmented distributed Bragg grating reflector with each segmentcapable of producing a peak corresponding to one of the peaks in thefirst comb, some at least of the segments in the segmented distributedBragg grating reflector being capable of modification so as to reflectlight at the wavelength of another peak so as to form a reinforced peak,so that the laser is capable of lasing at the wavelength of thereinforced peaks.

The segmented distributed Bragg grating reflector comprises a pluralityof segments, typically eight or nine, each comprising gratings, thedifferent segments having a grating which may be each of different pitchand their preferred organization being the longest pitch is at the facetend of the reflector, and the shortest pitch is closest to the gainsection, with progressive pitch size change therebetween.

The segments are modified to change the reflecting wavelength byaltering the refractive index of the material of which the segments areformed, and the refractive index may be varied by passing an electricalcurrent through the segments. Each segment has an associated electrodeto permit the passage of current through the segment arid each electrodeis actuable independently of the other electrodes.

The common method of controlling multi-section lasers is to provide onedigitally controlled current source per electrode and it is common toimplement each current source digitally using adigital-to-analogue-converter (DAC). However for a tunable laser havingmultiple sections and a plurality of independently actuable gratingsegments, to provide one digitally controlled DAC per section or segmenthas major disadvantages in circuit size, control complexity andconsequential costs.

It is an object of the present invention to provide a suitable controlfor a tunable laser having a segmented distributed Bragg gratingreflector, which ameliorates these problems.

STATEMENT OF THE INVENTION

According to the invention there is provided a control for a tunablelaser having a gain section, a comb reflection section, a phase changesection and a segmented Bragg grating reflector section comprising aseries of grating units each of a different pitch, and an electrodeassociated with each grating so that an electrical current is applicableto each individual grating, wherein the control includes a plurality ofcontrolled. sources of electrical current which are each connected toswitch means operable to independently connect each of said sources to arespective grating being one of a subset of consecutive gratingsselected from said series of grating units.

Each subset comprises ‘N’ gratings and each source may be connected to arespective switch means having a plurality of output terminals eachbeing respectively connectable to every Nth gratings in said series ofgrating units. The respective switch means preferably each comprise amulti-channel, preferably four channel, multiplexer. The multiplexersmay be each controlled by an embedded controllers, preferably amicro-processor, digital signal processor, or look-up table, operablevia an interface means; preferably a N-wire interface having a maximumof 2*n (two to the power of n) output channels, or a serial controlinterface with encoded address data, that defines the active path withinthe multiplexer.

Also according to the invention, there is provided a tunable laserhaving a gain section, a phase change section and a segmented Bragggrating reflector section comprising a series of grating units each of adifferent pitch, and an electrode associated with each grating so thatan electrical current is applicable to each individual grating, and acontrol according to the first aspect of the present invention, thecontrol activating at least one of a subset of consecutive gratings soas to reflect light at a wavelength of another peak to form a reinforcedpeak so that the laser is capable of lasing at the wavelength of thereinforced peak.

In this specification the term “light” will be used in the sense that itis used in optical systems to mean not just visible light but alsoelectromagnetic radiation having a wavelength between 700 nanometres(nm) and 3000 nm.

Preferably the gain section of the laser is bounded at one end by asegmented Bragg grating reflector and its other end by a phase changesection, which on its other side is, bounded by comb refection section,conveniently in the form of a distributed Bragg grating reflectorproducing a comb of reflective peaks.

Preferably, the distributed Bragg grating reflector is a phase gratingdistributed Bragg reflector of the type disclosed in GB 2337135.

Alternatively, the distributed Bragg grating reflector is asuperstructure grating reflector, of the type described in “Broad-RangeWavelength Tunable Superstructure Grating (SSG) DBR Lasers”, Y Thomoriet al., IEEE Photonics Technical Letters, Volume 5, No. 6, July 1993.

Alternatively, the distributed Bragg grating reflector is a sampledgrating Bragg reflector of the type described in U.S. Pat. No.4,896,325.

In a preferred embodiment, the Bragg grating segments are each keptshort so that the cumulative waveguide length associated with the Bragggratings is minimised, thereby keeping the waveguide attenuation losseslow. With short Bragg grating segments the associated reflectivity ofeach is broad and this causes a degree of overlap between neighbouringsegment reflectivity. However, by appropriate design the segmentreflectivity discrimination can be sufficient for stable lasingoperation in combination with reduced light power losses.

The grating comprising the distributed Bragg grating reflector is madelong so that its comb reflection peaks have finesse and constitute welldefined wavelengths. These comb wavelengths are typically centred onchannels in the communications band of interest.

Electrical current passing through the distributed Bragg reflectoralters the wavelengths at which the comb of reflecting peaks reflects.

Electrical current passing through the phase change section alters therefractive index of the material of the phase change section to effectthe phase and thereby minimise mode hoping.

Tunable lasers are'suitable for use in telecommunications systems, forexample, in the C-band, namely within the band 1530 nm to 1570 nm.

According to another aspect of the invention there is provided a methodof control for a tunable laser having a gain section, a phase changesection and a segmented Bragg grating reflector section comprising aseries of grating units each of a different pitch, with an electrodeassociated with each grating so that an electrical current is applicableto each individual grating to vary its refractive index, wherein saidmethod includes providing a plurality of controlled sources ofelectrical current, connecting the sources to switch means, andoperating the switch means to independently connect each of said sourcesto one of a subset of consecutive gratings selected from said series ofgrating units.

Preferably the laser is provide with a comb reflection section andpassing an electrical current through the comb reflection section altersthe wavelengths at which the comb of its reflecting peaks reflects.

Preferably, the laser emits light through the segmented distributedBragg grating reflector and end facet.

DESCRIPTION OF THE DRAWINGS

The invention will be described by way of example and reference to theaccompanying drawings in which:

FIG. 1 is a schematic cross section through a laser suitable foroperation in association with a control according to the presentinvention; and

FIG. 2 is a schematic drawing of a laser as shown in FIG. 1 having afirst control system according to the present invention; and

FIG. 3 is a schematic drawing of a laser as shown in FIG. 1 with asecond control system according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described by reference to a tunable laser as isdescribed in UK patent application 0118412.6, which will be describedherein only as is sufficient for an understanding of the presentinvention.

Common reference numbering has been used across all Figures for drawingelements with equivalent functionality.

Referring to FIG. 1, this shows a schematic cross sectional view of alaser in accordance with the present invention. As is conventional insemiconductor lasers the laser is built up in a series of layers, withan active layer 1 formed between a lower layer 2 and an upper layer 3.There will typically be a plurality of layers in the structure, but theyare not material to the invention and for better understanding of theinvention they are not shown. The active layer 1 also acts as awaveguide.

The laser itself has four principal sections. A gain section 4, a phasechange section 5 and front and rear reflecting section 6 and 7respectively. The rear reflecting section 7 has a phase gratingdistributed Bragg grating reflector 8 formed in the layer 3. Such areflector produces a comb of reflectance peaks at separated wavelengths,and each peak is of substantially the same amplitude. The frontreflector 6 is made up of a series of segments 9-17, each segment beinga distributed Bragg grating reflector unit, but each segment reflectingat nominally a single wavelength only. The wavelengths of the individualpeaks of the segments 9 to 15, and 17, corresponding to one of the peaksof the comb reflectance produced by the front reflector Bragg gratingreflector.

A common electrode 18 acts as the electrical return for thesemiconductor laser device.

Each of the sections 4, 5, 7, and segments 9-17 is provided with anassociated electrode 4 a, 5 a, 7 a, 9 a-17 a through which electriccurrent can be applied to the respective section or segment. Gratingunit 17 does not of necessity have to have an associated electrode 17 a,as its function is to form a lowest wavelength (band edge) down to whichthe next highest wavelength unit may be tuned.

The material of which layer 3 is formed in the sections corresponding tothe reflectors 6 and 7, is such as to experience a reduction inrefractive index when an electrical current is passed through it—theso-called electro-refractive effect. When light passes through a mediumof refractive index n_(eff) the actual wavelength of the light withinthat medium, which will be referred to herein as λ¹, is the wavelength λdivided by the value for the refractive index n_(eff). In other words:—λ¹ =λ/n _(eff)   (1)where n_(eff) is the effective refractive index of the medium as seen bythe propagating light of wavelength λ in free space.

Thus if, for example, a current is passed through the electrode 12 a,the wavelength at which the Bragg reflector in segment 12 reflects lightwill be reduced. If the reduction is such that it now reflects light atthe-same wavelength as the Bragg grating reflector of the adjacent lowerwavelength segment, segment 11, then the intensity of the peak forsegment 11 is reinforced such that it is much higher and more intensethan the other reflection peaks.

Now, however, if a current is passed through the electrode 4 a to createlight in the gain section 4, at an intensity above the lasing threshold,the light at the wavelength corresponding to the reinforced intensitysegment peak is preferentially operable and the laser will commence tolase at that wavelength. Thus the laser will be tuned to thatwavelength. If a current is now passed through electrode 7 a this willeffectively move the whole comb of peaks for that reflector to lowerwavelengths. If at the same time a current is passed through electrode11 a and the current passing through electrode 12 a are increased, thenthe reinforced peak will also move to a lower wavelength.

With reference now to FIG. 2, in order to supply an electrical currentto different sections and selected pairs of adjacent Bragg gratingsegments a control arrangement 20 is required. Control arrangement 20provides one digitally controlled current source 21, 22, 23 perelectrode 4 a, 5 a, 7 a and each current source is controlled digitallyusing a respective DAC 25. An exemplary DAC is supplied by AnalogDevices Inc., type AD5324, which is a quad 12 bit DAC.

Since in the segmented distributed Bragg reflector section 6, of FIG. 1,only two adjacent grating segments e.g. 11 and 12, are activated at anyone time this section is operated through only two current sources 31and 32, which are again each implemented using a respective DAC 25. Thetwo digitally controlled sources of electrical current 31 and 32 areeach connected to switch means 33. The switch means 33 is connected tothe electrodes 9 a-16 a and is operable to independently connect each ofsaid sources 31 and 32 to one of two adjacent gratings e.g. 11 and 12,selected from said series of gratings 9-16 A suitable switch 33 may be alinear commutator. The switch means 33 may be operated by a control 34,which one of ordinary skill will appreciate may be from a programmablecontroller (not shown) which also controls the DACs 25.

With reference now to FIG. 3, each current source 31 and 32 is connectedto one of a pair of 4-way multiplexers 35 and 36 respectively. As anexample, a suitable multiplexer is a MAX4634 available from MaximIntegrated Products. Each multiplexer 35, 36 has four outputs that aresingularly connected to alternate electrodes of the segmented Bragggrating section as shown in FIG. 3, i.e. 35 is connected to alternateelectrodes 9 a, 11 a, 13 a, 15 a and 36 is connected to alternateelectrodes 10 a, 12 a, 14 a and 16 a, so that pairs of adjacent segmentscan be concurrently activated with current. Each multiplexer 35, 36 iscontrolled via a 2-wire interface (A₀ and A₁) that defines the activepath within the multiplexer and hence which segment receives current.This interface may be provided with an embedded micro-controller 38. Tochange the laser wavelength and/or channel the micro-controllers 38configure the multiplexers according to Tables 1 and 2 below:—

TABLE 1 ACTIVE SWITCH O/P 1 3 5 7 A₀ L H L H A₁ L L H H

TABLE 2 ACTIVE SWITCH O/P 2 4 6 8 A₀ L H L H A₁ L L H H

-   -   In Table 1 and 2 L=Logical Low, H=Logical High, and output (O/P)        numbers correspond with those given on FIG. 3.

The two multiplexers therefore have unique switch settings for adjacentpairs of activated Bragg grating segments as is shown in Table 3 below:—

TABLE 3 MULTIPLEXER 35 MULTIPLEXER 36 PAIR A₀ A₁ A₀ A₁ 1 & 2 L L L L 2 &3 H L L L 3 & 4 H L H L 4 & 5 L H H L 5 & 6 L H L H 6 & 7 H H L H 7 & 8H H H H

-   -   In Table 3, L=Logical Low, H=Logical High

The multiplexer micro-processors 38, and the DACs 25, are controlled bya programmable controller, not shown, to produce the required operatingwavelength.

In many telecommunication applications the tunable laser will berequired to operate on ITU Grid channels for which a look-up table, notshown, may be used to set the control means to the required condition,and the laser thereby switched between channels. Alternativeapplications may require the laser to be continuously tunable for whichapplications the programmable controller may be dynamically driven usingvariable controls not shown, but of an obvious nature to those ofordinary skill in the art.

1. A control for a tunable laser having a first reflecting section, again section, a phase change section and a second reflecting sectioncomprising a segmented Bragg grating section having a series ‘P’adjacent grating units having a progressive pitch size change from oneend of the series to other end, and an electrode associated with eachgrating unit so that an electrical current is applicable to eachindividual grating unit, comprising a plurality of digitally controlledsources of electrical current which are each connected to a switchoperable to independently connect each of said plurality of sources to arespective one grating unit of a subset only of ‘N’ consecutive adjacentgrating units selected from said series of grating units, wherein ‘P’ isgreater than ‘N’, and ‘N’>1.
 2. A control as claimed in claim 1, whereinsaid ‘N’ grating units and each source of electric current is connectedto a respective switch having a plurality of output terminals each beingrespectively connectable to every Nth grating unit in said series ofgrating units.
 3. A control as claimed in claim 2, wherein said switchcomprise a multi-channel multiplexer.
 4. A control as claimed in claim3, wherein said multiplexer is controlled by an embedded controlleroperable via an interface that defines an active path within themultiplexer.
 5. A control as claimed in claim 2, wherein ‘N’ is two andNth is second.
 6. A control as claimed in any claim 1, comprising twodigitally controlled sources of electrical current, and wherein theswitch is operable to connect said sources to one of two adjacentgrating units forming a subset.
 7. A control a claimed in claim 6,wherein said switch comprises a four channel multiplexer.
 8. A tunablelaser comprising a first reflecting section, a gain section, a phasechange section, a second reflecting section comprising a segmented Bragggrating reflector having a series ‘P’ adjacent grating units having aprogressive pitch size change from one end of the series to other end,an electrode associated with each grating unit so that an electricalcurrent is applicable to each grating unit, and a control comprising aplurality of digitally controlled sources of electrical current whichare each connected to a switch operable to independently connect each ofsaid plurality of sources to a respective one grating unit of a subsetonly of consecutive grating units selected from said series of gratingunits, the control activating at least one of a pair of ‘N’ consecutiveadjacent grating units so as to reflect light at a wavelength of anotherpeak to form a reinforced peak so that the laser is capable of lasing atthe wavelength of the reinforced peak, wherein ‘P’ is greater than ‘N’,and ‘N’>1.
 9. A laser as claimed in claim 8, wherein the gain section isbounded at one end by the segmented Bragg grating reflector and at theother end by the phase change section, wherein an opposite side of thephase change section is bounded by a comb reflection section producing acomb of reflective peaks.
 10. A laser as claimed in claim 9, wherein thecomb reflection section comprises a phase grating distributed Braggreflector.
 11. A laser as claimed in claim 9, wherein the combreflection section comprises a sampled grating Bragg reflector.
 12. Alaser as claimed in claim 9, wherein the comb reflection sectioncomprises a superstructure grating reflector.
 13. A method of controlfor a tunable laser having a first reflecting section, a gain section, aphase change section and a second reflecting section comprising asegmented Bragg grating reflector having a series ‘P’ grating unitshaving a progressive pitch size change from one end of the series toother end, with an electrode associated with each grating unit so thatan electrical current is applicable to each individual grating unit tovary a refractive index, wherein said method comprises providing aplurality of digitally controlled sources of electrical current,connecting each of the plurality of sources to a switch, and operatingsaid switch to independently connect each of said sources to arespective grating unit being one of a subset only of ‘N’ adjacentgrating units selected from said series of grating units, wherein ‘P’ isgreater than ‘N’, and ‘N’>1.
 14. A method as claimed in claim 13,further comprising the laser providing at an end away from the segmentedBragg grating reflector section with a distributed Bragg gratingreflector wit an associated electrode, and passing an electrical currentthrough the distributed Bragg grating reflector to alter one or morewavelengths at which the comb of its reflecting peaks reflects.
 15. Amethod as claimed in claim 13, further comprising passing electriccurrent through the phase change section to alters a refractive index ofthe material of the phase change section to affect a phase and therebyminimize mode hoping.